Air pressure control in tires on performance vehicles, commercial fleet vehicles, rental car fleets, over the road vehicles and most any vehicle running on inflated tires can be extremely critical. Proper tire pressure decreases tire wear, improves vehicle mileage and increases vehicle safety. In a racing environment, tire pressure is extremely critical in optimizing vehicle performance.
A Tire Pressure Monitoring System (TPMS) is an electronic system designed to monitor the apparent air pressure inside all pneumatic tires on a motor vehicle. The system is sometimes referred to as Remote Tire Pressure Monitoring System (RTPMS), or simply as a run flat indicator. It transmits data on an unencrypted radio channel which can be intercepted by third parties and traced back to the owner of the vehicle.
The first passenger vehicle to adopt a TPMS was the Porsche 959 in 1986, using a system developed by PSK. Due to vehicle safety and maintenance economy, TPMS appeared more widely in Europe as an optional feature for top range luxury passenger vehicles, like the Audi A8, Mercedes-Benz S-Class and the BMW 7 Series. In 1999 the PSA Peugeot Citroen decided to adopt a TPMS as a standard feature on the Peugeot 607. The following year Renault launched the Laguna II, the first high volume mid-size passenger vehicle in the world to be equipped with a TPMS as a standard feature.
The Firestone recall in the United States in the late 1990s which was attributed to more than 100 deaths from rollovers following a tire tread-separation pushed the Clinton administration to publish the TREAD Act. This act mandates the use of a suitable TPMS technology in order to alert drivers of a severe under-inflation condition of their tires. This act affects all light motor vehicles (less than 10,000 lb) sold after Sep. 1, 2007.
Phase-in started in October 2005 at 20%, and reached 100% for models produced after September 2007. While in the United States, TPMS legislation grew from safety-related motives; European Union (EU) and Far East legislators are looking at TPMS as a way of reducing CO2 emissions, and are presently considering compulsory TPMS from this environmental stance.
Regardless of United States and EU legislation, the introduction by several tire manufacturers of nm-flat tires makes it mandatory for car manufacturers to fit a system where the drivers are made aware the run-flat has been damaged. The run-flats are designed to be used at no more than 80 km/h (50 mph) for no more than a distance of 80 km (50 miles), and this is why it is mandatory run-flats are monitored by TPMS. Run-flats have received a mixed reception from the public due to their impact on comfort with a harder ride.
Lastly, the most recent advance with TPMS technology is the introduction of battery-less direct sensor systems which require zero maintenance and are very reliable. VisiTyre is the first of this new class of battery-less TPMS which allows pressure on demand readings immediately from ignition and unlike radio frequency TPMS systems are also transparent to all tire construction types.
Direct sensor TPMS delivers real time tire pressure information to the driver of the vehicle—either via a gauge, a pictogram display, or a simple low pressure warning light. These systems employ physical pressure sensors inside each tire, and a means of processing and sending information from inside the tire to the vehicle instrument cluster.
Direct Sensor TPMS can identify simultaneous under inflation in all four tires or in any combination of tires. Direct sensor TPMS are designed to specifically cope with the effects of changes in tire pressure due to ambient temperature changes and road to tire friction based temperature changes. Friction between the tire and road surface heats up the tire and increases the pressure in the tire. Alarm activation threshold pressures are usually set according to the manufacturer's recommended “cold placard inflation pressures”.
A disadvantage of a battery powered radio frequency (RF) TPMS is the possibility the construction of certain steel belted radial replacement tires will block the very low power RF signal of the direct TPMS sensor. The power levels of battery powered TPMS are kept very low to conserve battery life and to conform to various countries communication authority standards for short range radio communications.
Another disadvantage of direct sensor TPMS is their physical sensors are quite large and they are either mounted on the end of valve sterns or by a steel band around a rim's dropwell center. In both cases these sensors are subject to damage during tire removal and fitting procedures. Banded sensors may also damage the tire bead's air seal.
Indirect acting TPMS measures the “apparent” air pressure indirectly, by monitoring individual wheel rotational speeds, and other signals available outside the tire itself. Most indirect TPMS use the fact an under-inflated tire has a slightly smaller diameter than a correctly inflated tire and therefore has to rotate at a higher angular velocity to cover the same distance as a correctly inflated tire. Such TPMS cannot detect under inflation in all four tires simultaneously, since if all four tires lose the same amount of air the relative change will be zero. In the United States the TREAD Act precludes the use of any TPMS which cannot simultaneously detect under inflation of any or all four tires.
Newer developments of indirect TPMS can also detect simultaneous under-inflation in all four tires thanks to vibration analysis of individual wheels or analysis of load shift effects during acceleration and/or cornering, however, additional sensors add to the complexity and cost of this technology. A hybrid method requires adding one direct TPMS sensor to a single tire on a vehicle, thereby eliminating the four equally low tire scenarios.
Indirect TPMS is cheap and easy to implement since most modern vehicles already have wheel speed sensors for anti-lock braking systems, and electronic stability control systems. Another advantage of this technology is no maintenance of the TPMS is necessary, such as purchasing anew sensor when the battery fails in a direct RF based TPMS.
A disadvantage of an indirect system, compared to direct, is also the periodic requirement for recalibration, creating ‘unsafe’ windows where low tire detection is unavailable. Recalibration must be started manually when tires are changed or re-inflated. Forgetting to perform this initialization can lead to potentially dangerous false-positive or false-negative alerts. Incorrect calibration by the user (such as calibration when one or more tires are underinflated) can also cause unreliable operation.